Active vibration control system with non-concentric revolving masses

ABSTRACT

A vibration control system for a rotor hub provides vibration attenuation in an aircraft by reducing the magnitude of rotor induced vibratory. The system can include a force generating device attached to a rotor hub which rotates along with the rotor at the rotational speed of the rotor. Vibratory shear force is generated by rotating unbalanced weights each about an axis non-concentric with the rotor hub axis at high speed to create large centrifugal forces. The rotational speed of the weights can be a multiple of the rotor rotational speed to create shear forces for canceling rotor induced vibrations. The amplitude of the generated shear force is controlled by indexing the positions of the unbalanced weights relative to each other, while the phase of the shear force is adjusted by equally phasing each weight relative to the rotor.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/600,180, filed Jan. 20, 2015, which claims priority to U.S.provisional application No. 61/930,004, filed Jan. 22, 2014, thedisclosures of which are hereby incorporated by references for allpurposes as if fully set forth herein.

BACKGROUND Technical Field

The present disclosure relates to vibration attenuation for rotor hubs.

Description of Related Art

Rotary-wing aircraft, such as helicopters and tiltrotors, have at leastone rotor for providing lift and propulsion forces, and these rotorshave at least two airfoil blades connected to a rotatable hub. Theblades cause vibrations that are a function of the rotational speed ofthe rotor, and aircraft designers have difficulty accurately predictingthe exact vibration modes that a particular rotor configuration willencounter. The vibrations can be transmitted through the rotor hub,through the rotor mast, through associated powertrain components, andinto the airframe of the aircraft. The vibrations can reduce the life ofaffected components and cause undesirable vibration for passengers.Various types of vibration attenuation systems have been developed toreduce or eliminate these vibrations. The prior art includes bothpassive and active devices that are airframe-mounted, mounted at theinterface between the airframe and the rotor system, and devices mountedin the rotor system.

Conventionally, active control of helicopter vibration has been by oneof three methods; installing force generating actuators onto theairframe, or installing actuation devices between the main rotor pylonassembly and the airframe, or installing actuation devices in the mainrotor system. Active vibration control systems, for rotorcraftapplications, are closed loop control systems designed to suppressvibrations at harmonics of the main rotor frequency. Typically, arotorcraft active vibration control system is comprised of vibrationsensors; a control computer to process the vibration measurements andoutput control commands to an actuation device; and actuation devices toproduce force inputs to the rotorcraft airframe, or at thepylon-airframe interface, or actuation inputs to the rotor hub, oractuation inputs to the rotor blades. Vibration reduction is achieved bysuperposition of the vibrations created by the active system's actuationforces, and the vibrations caused by the rotorcraft main rotor system.

Active vibration control systems have been proposed that are mounted tothe main rotor hub and apply control forces to the main rotor hub. Thesetypes of systems propose two or more pairs of masses revolving about andconcentric with the center of the main rotor mast to create acontrollable force to cancel the rotor generated forces.

Although great strides have been made in the art of vibrationattenuators for rotorcraft, there is an on-going need for vibrationsuppression devices and methods that are more effective, require lessweight, require less power, and have less severe failure modes.

DESCRIPTION OF THE DRAWINGS

The novel features believed characteristic of the system and method ofthe present disclosure are set forth in the appended claims. However,the system and method itself, as well as a preferred mode of use, andfurther objectives and advantages thereof, will best be understood byreference to the following detailed description when read in conjunctionwith the accompanying drawings, wherein:

FIG. 1 is an oblique view of an aircraft having an active vibrationcontrol system, according to one example embodiment;

FIG. 2 is an oblique, partially sectioned view of a proprotor of theaircraft of FIG. 1, according to one example embodiment;

FIG. 3 is a schematic top view of a portion of the active vibrationcontrol system of the aircraft of FIG. 1, according to one exampleembodiment;

FIG. 4 is a schematic top view of a portion of the active vibrationcontrol system of the aircraft of FIG. 1, according to one exampleembodiment;

FIG. 5 is a schematic top view of a portion of the active vibrationcontrol system of the aircraft of FIG. 1, according to one exampleembodiment;

FIG. 6 is a schematic side view of a portion of the active vibrationcontrol system mounted to a standpipe that is enclosed by and coaxialwith a rotor mast, according to another example embodiment;

FIG. 7 is a schematic top view of a portion of the active vibrationcontrol system, according to another example embodiment;

FIG. 8 is a schematic side view of a portion of the active vibrationcontrol system, according to another example embodiment; and

FIG. 9 is a schematic side view of a portion of the active vibrationcontrol system, according to another example embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Illustrative embodiments of the system and method of the presentdisclosure are described below. In the interest of clarity, all featuresof an actual implementation may not be described in this specification.It will of course be appreciated that in the development of any suchactual embodiment, numerous implementation-specific decisions must bemade to achieve the developer's specific goals, such as compliance withsystem-related and business-related constraints, which will vary fromone implementation to another. Moreover, it will be appreciated thatsuch a development effort might be complex and time-consuming but wouldnevertheless be a routine undertaking for those of ordinary skill in theart having the benefit of this disclosure.

In the specification, reference may be made to the spatial relationshipsbetween various components and to the spatial orientation of variousaspects of components as the devices are depicted in the attacheddrawings. However, as will be recognized by those skilled in the artafter a complete reading of the present disclosure, the devices,members, apparatuses, etc. described herein may be positioned in anydesired orientation. Thus, the use of terms such as “above,” “below,”“upper,” “lower,” or other like terms to describe a spatial relationshipbetween various components or to describe the spatial orientation ofaspects of such components should be understood to describe a relativerelationship between the components or a spatial orientation of aspectsof such components, respectively, as the device described herein may beoriented in any desired direction.

An active vibration control system for a rotor hub provides vibrationattenuation in a rotary-wing aircraft by reducing the magnitude of rotorinduced vibratory forces acting on the airframe. The active vibrationcontrol system includes a force generating device attached to a rotorhub and rotates along with the rotor at the rotational speed of therotor. Within the force generating device, vibratory shear force isgenerated by rotating pairs of unbalanced weights at high speed tocreate large centrifugal forces, and the unbalanced weights may bedriven by electric motors or by torque provided by the rotor mast. Therotational speed of the unbalanced weights will typically be a multipleof the rotor rotational speed to create shear forces for canceling rotorinduced vibrations, which can be rotating in the same direction as therotor or in the opposite direction. The amplitude of the generated shearforce is controlled by indexing the positions of the unbalanced weightsrelative to each other, while the phase of the shear force is adjustedby equally phasing each unbalanced weight relative to the rotor. Amicroprocessor-based control system uses feedback from vibration sensorsto command the operation of the unbalanced weights so as to minimizevibrations transmitted to the airframe.

This system is an improvement over methods now being used because it islighter weight, more compact, and is capable of better vibrationreduction. The principal advantage of this device is that it cancels thesource of vibratory loads, thereby reducing vibration throughout theentire aircraft. By reducing the magnitude of rotor-induced vibratoryloads, the active vibration control system can improve the fatigue lifeof critical structural components, reduce vibration of avionics, reduceengine vibration, and improve passenger comfort.

FIG. 1 is an oblique view of a rotary-wing aircraft having an activevibration control system, which is described herein. Aircraft 11 is arotary-wing aircraft, specifically a tiltrotor aircraft, having afuselage 13 and wings 15 extending from fuselage 13. Fuselage 13 andwings 15 comprise the airframe of aircraft 11. A rotatable nacelle 17 islocated at the outer end of each wing 15 for housing an engine (notshown), and each engine is configured for providing torque to causerotation of an attached proprotor 19. Each proprotor 19 has a pluralityof blades 21, which are connected to a hub (see FIG. 2) located beneathan aerodynamic fairing, referred to as a spinner 23.

FIG. 2 is an oblique view of a proprotor 19 with blades 21 removed fromyoke 25 of the hub. Holes 27 are formed in spinner 23 (a portion iscutaway for ease of viewing) for allowing portions of yoke 25 toprotrude for attachment of blades 21. A mast 29 is connected to anoutput of the engine for transfer of torque from the engine to mast 29.In the configuration shown, a constant-velocity drive assembly 31 issplined to mast 29 for rotation with mast 29, and yoke 25 is connectedto drive assembly 31. Drive assembly 31 allows for yoke 25 to gimbalrelative to mast 29 as mast 29 drives yoke 25 in rotation about mastaxis 33.

In the configuration shown, an active vibration control system 37 iscarried on an end portion of mast 29. Active vibration control system 37contains a force generator 39, and a microprocessor-based controller 41to command the force generator 39. Force generator 39 and controller 41are splined or otherwise affixed to mast 29 for rotation with mast 29.In the embodiment illustrated in FIG. 3, the force generator 39 containsat least two, and preferably four rotating unbalanced weights 50. Theaxis of rotation of each unbalanced weight 50 is non-concentric withmast axis 33, but is parallel to the mast axis 33 and offset from mastaxis 33 a fixed radial distance. Unbalanced weights 50 are configuredand commanded in pairs with each weight's axis of rotation symmetricallyopposed about mast axis 33. Each unbalanced weight 50 has a center ofmass that is located a radial distance from the axis of rotation forthat particular weight 50, and driven in rotation about said axis. Eachunbalanced weight 50 can be driven by a respective motor 52. In oneembodiment, each motor 52 is an electric motor; however, in alternativeembodiments motors 52 can be hydraulic or pneumatic, for example.Rotation of each unbalanced weight 50 about its particular axis andcombined with rotation of the force generator 39 about mast axis 33causes an oscillatory shear force on mast 29 in the plane of rotation.The amplitude of the force generator 39 shear force output is controlledby the rotational speed of the unbalanced weight 50, and indexing thepositions of each unbalanced weight 50 relative to each other, while thephase of the shear force output is adjusted by equally indexing eachunbalanced weight 50 relative to proprotor 19. The rotational speed ofthe unbalanced weights 50 is a multiple of the proprotor 19 rotationalspeed to create shear forces for canceling rotor induced vibrations,which can be rotating in the same direction as the proprotor 19 or inthe opposite direction.

Referring also to FIGS. 1 and 2, controller 41 is located at the rotorhub in close proximity to the force generator 39 and is configured toautomatically control the operation of the mast mounted force generator39. Alternatively, the microprocessor-based controller 41 can be locatedin the fuselage 13, the wings 15, or the nacelles 17. Controller 41preferably comprises feedback sensors, such as sensors 45 located on therotor mast, on fuselage 13, on wings 15, and on nacelles 17 to providevibration feedback data. Though shown in particular locations, sensors45 may be installed in other locations. Use of sensors 45 allowscontroller 41 to control the operation of the mast mounted forcegenerator 39 based on measurements of vibrations transmitted into andthrough the airframe. Controller 41 may alternatively control operationof force generator 39 based on other data, such as airspeed, rotorspeed, blade pitch angle, nacelle angle, amount of rotor thrust, and/orsimilar aircraft parameters.

Operational control of force generator 39 preferably includes commandingat least rotational speed, rotational direction, and indexing of theunbalanced weights 50, and phasing of the unbalanced weights 50 relativeto proprotor 19. Controller 41 and force generator 39 may be providedwith “fail-off” features to prevent active vibration control system 37from inducing unintended and undesirable vibrations in the event offailure of one or more components of the system. Inputs to controller 41may include aircraft gross weight, load factor, altitude, airspeed, androtor speed (RPM). In addition, controller 41 may be optimized for useon tiltrotor aircraft 11 by also basing commands on the angle ofnacelles 17 and other tiltrotor-specific parameters.

Controller 41 independently commands each unbalanced weight 50 to bedriven in the selected rotational direction and at the selectedrotational speed. For example, the unbalanced weights 50 may be drivenin the same rotational direction as mast 29 and at a multiple of therotational speed of mast 29. The unbalanced weights 50 createoscillatory shear forces in the plane of rotation at a frequencydescribed as the number of cycles per revolution of proprotor 19(n/rev). When the shear forces are equal in amplitude to the aerodynamicn/rev forces of proprotor 19 and opposite their phase, then no vibratoryforce will be transmitted to the airframe. For example, if a four-bladeproprotor 19 is rotating at 400 revolutions per minute, and the forcegenerator 39 is to oppose 4/rev vibrations by rotating unbalancedweights 50 in the direction of proprotor 19, controller 41 will causethe weights to spin at 4× the speed of proprotor 19 relative to theairframe. Because mast 29 is spinning in the same direction as forcegenerator 39 relative to the airframe at 1× the speed of proprotor 19,the weights 50 will be spinning at 3× the speed of proprotor 19 relativeto mast 29 and proprotor 19. Likewise, if force generator 39 is tooppose 8/rev vibrations by rotating unbalanced weights in the oppositerotation of proprotor 19, controller 41 will cause the weights 50 tospin at 8× the speed of proprotor 19 relative to the airframe. Becausemast 29 and force generator 39 are spinning in the same direction at 1×the speed of proprotor 19, the unbalanced weights 50 will be spinning inthe opposite direction at 9× the speed of proprotor 19 relative to mast29 and proprotor 19.

The magnitude of the total oscillatory shear force produced by forcegenerator 39 is determined by the relative position of the center ofmass of each unbalanced weight 50. FIGS. 3, 4, and 5 illustrate therelative rotational positions of each unbalanced weight 50 of forcegenerator 39 for three modes of operation, with each figure viewed alongmast axis 33. In each figure, the direction of rotation of mast 29 isshown by arrow 47, and the direction of rotation of each unbalancedweight 50 is shown by arrow 49.

As described above, each unbalanced weight 50 has a center of masslocated a radial distance from its axis of rotation, and this may beaccomplished, for example, by locating a mass 51 along a peripheralportion of a disk 53. Mass 51 may be formed as an integral portion ofdisk 53 or may be formed as a separate component and attached to disk53. To provide for additional tuning of force generator 39, each mass 51may be configured to be replaceable, for example, by a similarlyconstructed mass 51 having more or less mass. Mass 51 may also beconstructed of multiple pieces, allowing mass 51 to be adjusted byremoving or adding pieces. Though shown as having only one mass 51, itshould be understood that disks 53 may configured to have more than mass51. Alternatively, mass 51 may be adapted to the axis of rotation by oneor more arms extending from the axis of rotation.

If masses 51 of force generator 39 are diametrically opposed, as shownin FIG. 3, while disks 53 are driven in rotation at the same speed, thenthe amplitude of the vibratory force will be zero. This is due to thefact that each disk 53 causes an equal and opposite shear force thatcancels the force caused by the diametrically opposed disk of forcegenerator 39. If disks 53 are indexed during rotation so that masses 51are aligned, as shown in FIG. 4, the total shear force is the maximummagnitude that force generator 39 can produce for any given rotationalspeed. Any magnitude between zero and the maximum is available bychanging the relative rotational positions of disks 53, and FIG. 5 showsdisks 53 as having been indexed at an angle of approximately 45 degreesrelative to the index positions of FIG. 3.

Proprotor 19 is described as having only one force generator 39, thoughadditional force generators may be added to oppose additional vibrationmodes (8/rev, 12/rev, etc.). Additional force generators are added in acoaxial arrangement along mast axis 33, and each force generator maycomprise weights having a different weight than disks 53 and operatingat a selected rotational speed different than disks 53. It should benoted that the force generators will be different for different types ofrotors, as the weights and axes of rotation will be optimized for theparticular application.

Another feature of the active vibration control system described aboveis the capability to provide some vibration attenuation when thecontroller 41 is not commanding the unbalanced weights to be driven inrotation. If the unbalanced weights are not driven in rotation, theweights will freely respond to rotor hub motions in a manner analogousto a simple pendulum absorber response. The dynamic response of thenon-driven unbalanced weights creates oscillatory shear forces at therotor hub for reduction of rotor induced vibrations. This feature of theactive vibration control system provides a fail-safe mode of operationin the event a component or system failure prevents the unbalancedweights 50 from being driven in a prescribed manner.

Another feature of the active vibration control system described aboveis the absence of a 1/rev mast load when the controller 41 is notcommanding the unbalanced weights 50 to be driven in rotation. If theunbalanced weights 50 are not driven in rotation, the weights 50 will beforced to a steady position by the centrifugal force caused by the 1/revrotation of the force generator 39. In this position the center of massof each weight 50 will be at the same radial distance from the mast axis33. Since the weights 50 are symmetrically positioned within forcegenerator 39, the total center of mass of force generator 39 will becoincident with the mast axis, and a 1/rev mast load will not becreated.

Other embodiments of the active vibration control system force generator39 described above may include a gear-type drive system for driving theunbalanced weights in rotation rather than using electric, hydraulic, orpneumatic motors. This type of force generator would operate withoutrequiring a large external source of power, as the power required foroperation is preferably taken from the mast. Motors 52 would then besmall electric motors or clutches to position and index the unbalancedweights relative to each other and relative to the proprotor forphasing, but once indexed and phased; the parasitic power requirement isnegligible and is derived from the mast torque.

Another feature that may be incorporated in the active vibration controlsystem described above is a “standpipe” configuration for mounting ofthe force generator. FIG. 6 shows an example embodiment, in which a mast95 encloses a coaxial standpipe 97. In FIG. 6, mast 95 is shown with aportion removed for ease of viewing standpipe 97. Mast 95 rotatesrelative to the airframe (not shown) about axis 99 for rotating anattached proprotor (not shown). Standpipe 97 is stationary relative tothe airframe, and bearings 101 are located between an outer surface ofstandpipe 97 and an inner surface of mast 95 to allow for the relativemotion of mast 95 relative to standpipe 97. In the embodiment shown, aforce generator 109 and controller 107 are mounted to a narrowed section111 at an outer end of standpipe 97. In operation, motors 52 rotate theunbalanced weights within force generator 109 in a similar manner asthose described above, allowing force generator 109 to produceoscillatory shear forces on standpipe 97. These shear forces are thentransferred into mast 95 through bearings 101. It should be noted thatmore force generators than is shown may be mounted on standpipe 97. Itshould also be noted that a standpipe configuration is particularlyuseful with the gear-type drive system described above.

Another example embodiment of the active vibration control system forcegenerator 39 is illustrated in FIG. 7. Force generator 39 can contain atleast one pair, and preferably two pair of rotating unbalanced weights50. The axis of rotation of each unbalanced weight 50 is non-concentricwith mast axis 33, but is parallel to the mast axis 33 and offset frommast axis 33 a fixed radial distance. Unbalanced weights 50 areconfigured and commanded in pairs with the axis of rotation of eachunbalanced weight 50 in a pair being diametrically opposed about mastaxis 33. Any number of pairs of unbalanced weights 50 can be utilized aslong as the axis of rotation for each unbalanced weight 50 is offsetfrom mast axis 33 the same radial distance, and the axes of rotation foreach unbalanced weight 50 in a pair are diametrically opposed about mastaxis 33. The relative position of one pair of unbalanced weights 50 toanother pair of unbalanced weights 50 is not constrained to a particularrelationship, as long as the axis of rotation for each unbalanced weight50 is offset from mast axis 33 the same radial distance, and the axes ofrotation for each unbalanced weight 50 in a pair are diametricallyopposed about mast axis 33. Operational control of force generator 39 issimilar to the methods further described herein.

Another example embodiment of the active vibration control system forcegenerator 39 is illustrated in FIG. 8. Force generator 39 can contain atleast one pair, and preferably two pair of rotating unbalanced weights50, but only one pair of unbalanced weights 50 is shown for clarity. Theaxis of rotation of each unbalanced weight 50 is normal to mast axis 33and offset from mast axis 33 a fixed radial distance. Unbalanced weights50 are configured and commanded in pairs with each weight's axis ofrotation symmetrically opposed about mast axis 33. Each unbalancedweight 50 has a center of mass that is located a radial distance fromthe axis of rotation for that particular unbalanced weight 50, anddriven in rotation about said axis. Rotation of each unbalanced weight50 about its particular axis and combined with rotation of the forcegenerator 39 about mast axis 33 causes an oscillatory shear force onmast 29. Operational control of force generator 39 is similar to themethods further described herein.

Another example embodiment of the active vibration control system forcegenerator 39 is illustrated in FIG. 9. Force generator 39 contains atleast one pair, and preferably two pair of rotating unbalanced weights50, but only one pair of unbalanced weights 50 is shown for clarity. Theaxis of rotation of each unbalanced weight 50 is normal to mast axis 33and the axis projection intersects mast axis 33. Unbalanced weights 50are offset from mast axis 33 a fixed radial distance. Unbalanced weights50 are configured and commanded in pairs with each weight having acommon axis of rotation. Each unbalanced weight 50 has a center of massthat is located a radial distance from the axis of rotation for thatparticular unbalanced weight 50, and driven in rotation about said axis.Rotation of each unbalanced weight 50 about its particular axis andcombined with rotation of the force generator 39 about mast axis 33causes an oscillatory shear force on mast 29. Operational control offorce generator 39 is similar to the methods further described herein.

The active vibration control system described above provides for severaladvantages, including: (1) improved capability of vibration attenuation;(2) attenuation of vibration at the mast before transmission to theairframe; (3) passive vibration attenuation if the unbalanced weightsare not driven in rotation; (4) reduced weight; and (5) improvedreliability.

This description includes reference to illustrative embodiments, but itis not intended to be construed in a limiting sense. Variousmodifications and combinations of the illustrative embodiments, as wellas other embodiments, will be apparent to persons skilled in the artupon reference to the description. For example, embodiments of activevibration control systems are shown installed on four-blade tiltrotorproprotors, though embodiments of active vibration control systems maybe used on a tiltrotor proprotor having any number of blades and anyother type of rotor, such as a helicopter rotor or aircraft propeller.

Embodiments of the active vibration control system can include one ormore computer systems having hardware and software for performing one ormore tasks described herein. This can include, for example, a computerhaving one or more processing units and non-volatile memories that storenon-transitory software instructions for instructing the processingunits to perform at least some of the tasks described herein. Further,the software described herein is non-transitory.

The particular embodiments disclosed above are illustrative only, as thesystem may be modified and practiced in different but equivalent mannersapparent to those skilled in the art having the benefit of the teachingsherein. Modifications, additions, or omissions may be made to theapparatuses described herein without departing from the scope of theinvention. The components of the system may be integrated or separated.Moreover, the operations of the system may be performed by more, fewer,or other components.

Furthermore, no limitations are intended to the details of constructionor design herein shown, other than as described in the claims below. Itis therefore evident that the particular embodiments disclosed above maybe altered or modified and all such variations are considered within thescope and spirit of the application. Accordingly, the protection soughtherein is as set forth in the claims below.

To aid the Patent Office, and any readers of any patent issued on thisapplication in interpreting the claims appended hereto, applicants wishto note that they do not intend any of the appended claims to invokeparagraph 6 of 35 U.S.C. § 112 as it exists on the date of filing hereofunless the words “means for” or “step for” are explicitly used in theparticular claim.

The invention claimed is:
 1. An active vibration control system for aproprotor of an aircraft, the proprotor including a plurality of bladesconnected to a rotor hub rotated by a mast about a mast axis ofrotation, the active vibration control system comprising: a forcegenerator attached to the mast for rotation with the mast, the forcegenerator comprising: (a) a first motor; (b) a second motor; (c) a firstweight configured to be independently driven about a first axis ofrotation by the first motor, the first weight disposed on a first armextending away from the mast; (d) a second weight configured to beindependently driven about a second axis of rotation by the secondmotor, the second weight disposed on a second arm extending away fromthe mast; (e) a third weight independently driven about a third axis ofrotation by a third motor and being diametrically opposed to the firstweight; and (f) a fourth weight independently driven about a fourth axisof rotation by a fourth motor and being diametrically opposed to thesecond weight; wherein the first weight and the third weight form afirst set, and the second weight and the fourth weight form a secondset; and wherein the first axis of rotation and the second axis ofrotation are non-concentric and parallel to the mast axis of rotation; acontroller for receiving input signals and outputting command signalsthat are at least dependent on a rotational speed of the proprotorindependently to the first motor, the second motor, the third motor, andthe fourth motor to control a generated oscillatory shear force of theforce generator; and a sensor for measuring vibration and providinginput to the controller; wherein during operation the first set isrotated about the mast axis of rotation at a different rotational speedthan the second set, allowing the generated oscillatory shear force ofthe force generator to attenuate vibrations at multiple frequencies;wherein during operation the first set is rotated about the mast axis ofrotation in an opposite rotational direction than the second set;wherein the axis of rotation for each of the first weight, the secondweight, the third weight, and the fourth weight are offset from the mastaxis a same radial distance; and wherein a first relative position ofthe first weight and the third weight in the first set and a secondrelative position of the second weight and the fourth weight of thesecond set have zero additional positional relationship constraintsother than being the same radial distance from the mast axis and beingdiametrically opposed from one another.
 2. The active vibration controlsystem according to claim 1, wherein during operation the first weightabout the first axis of rotation and the second weight about the secondaxis of rotation share a same rotational direction as the mast about themast axis of rotation.
 3. The active vibration control system accordingto claim 1, wherein the third axis of rotation and the fourth axis ofrotation are non-concentric and parallel to the mast axis of rotation,and during operation the third weight about the third axis of rotationand the fourth weight about the fourth axis of rotation share a samerotational direction as the mast about the mast axis of rotation.